GQD Anthorn: Britain's submarine transmitter on 19.58 kHz

What you hear

On 19.58 kHz, a strong, continuous signal that looks on the waterfall like a bright ribbon a few hundred hertz wide and sounds, in CW mode, like a steady tone with a rough, hissing grain. There are no pauses, no identification a listener could catch, and no change from hour to hour. This is GQD, the British Royal Navy's VLF transmitter at Anthorn on the Solway coast of Cumbria, and after DHO38 in Germany it is one of the strongest signals in the European very-low-frequency band.

The grain in the tone is minimum-shift keying, MSK, a narrow and efficient form of frequency-shift data. The stream is encrypted and carries naval traffic, so there is nothing for a decoder to do. What a listener gets is the pleasure of finding it: a signal so low in frequency that its wavelength is over fifteen kilometres, pulled out of a band that most receivers never visit.

Who transmits and why

Submerged submarines are deaf to ordinary radio; sea water stops shortwave and VHF almost at the surface. Very long waves go a few metres down, which is enough for a boat near the surface or trailing a wire antenna to receive without showing itself. Navies that operate submarines therefore keep VLF transmitters running day and night, and Anthorn is the British one. Its antenna is a vast arrangement of masts and wires on the flat Solway shore, built for a wavelength that no ordinary mast could serve.

The same site has a second life that listeners know well: since 2007 it has carried the MSF time signal on 60 kHz, the pulses that set radio-controlled clocks across Britain. The two signals are unrelated in purpose and separated by a wide gap in frequency, but they come from the same patch of ground, and a receiver that hears one will usually hear the other.

The content of the 19.58 kHz stream is a military secret and will stay one. Its existence is not: the station transmits continuously, in a published band, and listening to it is only receiving.

When and where to listen

VLF travels between the ground and the lower ionosphere and is remarkably stable, steady by day and stronger but more variable at night, with a brief wobble at dawn and dusk. GQD is strong across Britain, Ireland, France, the Low Countries and Scandinavia at any hour, and readable much further on a quiet receiver. The receiver is the limiting factor: the antenna must work this low, and the site must be free of the mains harmonics and switching noise that fill the VLF band near towns. A rural receiver in Britain or Ireland is ideal, one in the Netherlands or Germany nearly as good.

The transmitter is occasionally off for maintenance; when it is, the band near 19.6 kHz looks oddly bare. The program tunes the station in CW with a passband of about a kilohertz, placing the stream as a tone in the middle of the audio. If you want to compare it with DHO38, both fit on one waterfall: they are less than 4 kHz apart.

In ShackDeck

Station catalogue → Mystery stations → GQD Anthorn (19.58 kHz). The program picks a public receiver whose range reaches 19.58 kHz, tunes it and sets CW; no receiver, antenna or account of your own is needed. On a receiver that hears VLF well, the ribbon of GQD and the brighter ribbon of DHO38 share the screen, with the rest of Europe's VLF transmitters scattered between and below them.